Cancer is a broad group of diseases characterised by abnormal cells that acquire the capacity for uncontrolled proliferation, survival, invasion of surrounding tissues and, in many cases, spread to distant sites. Cancer is not one disease: tumours differ substantially according to tissue of origin, genetic alterations, molecular pathways, growth behaviour and response to treatment. Malignant neoplasms are distinguished from benign tumours primarily by their ability to invade locally and/or metastasise, although biological behaviour varies among cancers. Cancer can arise in virtually any tissue and includes carcinomas, sarcomas, haematological malignancies and other specialised tumour types. The disease process results from accumulated genetic and epigenetic alterations that disrupt normal controls over cell division, differentiation, DNA repair, apoptosis and interactions with surrounding tissues.
Cancer develops through multistep biological processes in which cells acquire alterations that confer selective advantages. Oncogenes can become abnormally activated, while tumour-suppressor genes can be lost or inactivated; defects in DNA-repair pathways can increase mutation accumulation. Not every mutation in a cancer cell causes the disease: driver alterations contribute to malignant behaviour, while passenger alterations may accompany tumour evolution without being essential to it.
Clonal evolution allows genetically or epigenetically distinct populations within a tumour to emerge under selective pressures such as immune surveillance or treatment. This heterogeneity helps explain why some tumours respond unevenly to therapy and why resistance can develop. Cancer therefore represents an evolving population of abnormal cells rather than a static mass with uniform biological properties.
Risk factors vary by cancer type and include tobacco exposure, alcohol, ultraviolet radiation, certain infections, obesity, environmental carcinogens, ionising radiation, inherited genetic predisposition and increasing age. Human papillomavirus contributes to several cancers, while hepatitis B and C viruses contribute to liver cancer risk and Helicobacter pylori infection is associated with gastric cancer. Lifestyle factors can influence risk, but cancer can also occur in people without obvious modifiable risk factors.
Inherited pathogenic variants in genes such as BRCA1, BRCA2 and mismatch-repair genes can substantially increase risk for particular cancers, but most cancers are not inherited directly. Prevention therefore involves a combination of vaccination, tobacco control, occupational and environmental protection, healthy behaviours, appropriate screening and management of inherited risk where relevant.
Cancer screening aims to detect disease or precancerous changes before symptoms develop, but screening is appropriate only when evidence supports a favourable balance of benefits and harms. Examples include screening programmes for colorectal, breast and cervical cancer in defined populations, with exact eligibility and testing intervals depending on national guidance. Screening differs from diagnostic investigation because screening targets people without relevant symptoms, whereas diagnostic testing evaluates symptoms or findings that already exist. Potential harms include false-positive results, overdiagnosis, unnecessary procedures and anxiety. Screening therefore cannot simply be assumed to improve outcomes because it detects more abnormalities; the crucial question is whether earlier detection reduces clinically important morbidity or mortality.
Diagnosis usually requires integration of clinical assessment, laboratory investigations, imaging and pathology. Imaging can identify a mass, determine its anatomical extent and assess possible spread, but histopathological examination frequently establishes the definitive tumour type. Molecular testing can identify genetic alterations, receptor status or other biomarkers that influence classification, prognosis or treatment. Staging describes the extent of cancer, commonly considering the primary tumour, regional lymph nodes and distant metastases, although staging systems differ among tumour types. Grading describes characteristics of tumour cells and architecture associated with biological aggressiveness and is distinct from staging. Accurate diagnosis and staging are essential because treatment decisions depend heavily on tumour biology and extent of disease.
Cancer treatment can include surgery, radiotherapy, chemotherapy, endocrine therapy, targeted therapy, immunotherapy, stem-cell transplantation for selected malignancies and combinations of these approaches. Surgery can remove localised tumours or reduce tumour burden, while radiotherapy uses ionising radiation to damage cancer-cell DNA in a defined treatment field. Chemotherapy generally uses cytotoxic drugs that interfere with cell division or other essential cellular processes and can affect normal rapidly dividing tissues.
Targeted therapies act on particular molecular pathways, while immunotherapies modify or enhance immune responses against cancer. Treatment can be curative, adjuvant, neoadjuvant, disease-controlling or palliative depending on the cancer and clinical situation. The distinction between palliative treatment and "giving up" is important: palliative care can be provided at any stage and aims to relieve symptoms and improve quality of life alongside disease-directed treatment.
Cancer can metastasise when malignant cells invade surrounding tissue, enter blood or lymphatic vessels, survive transport, exit into distant tissues and establish secondary tumour growth. Metastatic disease is biologically complex because only a subset of tumour cells may successfully complete each stage of the metastatic process. Common metastatic sites vary by primary tumour, and metastatic disease can cause organ-specific complications such as bone pain, neurological deficits, liver dysfunction or respiratory compromise. Cancer-associated symptoms can also arise from systemic effects, treatment toxicity or inflammatory responses rather than direct tumour invasion. Management of advanced cancer may combine tumour-directed therapy with symptom control, rehabilitation, nutritional support, psychological support and palliative care.
The modern concept of cancer developed from centuries of observations of abnormal growth, followed by advances in microscopy, pathology, genetics, molecular biology, imaging and therapeutics. Major developments include surgical anaesthesia and antisepsis, radiotherapy, cytotoxic chemotherapy, molecularly targeted drugs and immunotherapy. Cancer mortality and survival vary substantially by tumour type, stage, healthcare access and treatment effectiveness.
A key misconception is that cancer is inevitably fatal; many cancers are curable, particularly when detected and treated at an appropriate stage, while others can be controlled for prolonged periods. Another misconception is that one universal "cure for cancer" is realistic, because cancers have diverse biological mechanisms and require different treatments. Contemporary oncology therefore integrates prevention, early detection, precise diagnosis, molecular classification, multimodal treatment, survivorship care and palliative medicine.
